The present invention relates to devices, systems and methods for the characterization of cardiac rhythm and, particularly, characterization of ventricular fibrillation, and to devices, systems and methods for use in the treatment of ventricular fibrillation based upon the characterization of ventricular fibrillation.
References set forth herein may facilitate understanding of the present invention or the background of the present invention. Inclusion of a reference herein, however, is not intended to and does not constitute an admission that the reference is available as prior art with respect to the present invention.
Ventricular fibrillation (VF) is a leading cause of sudden death. Indeed, ventricular fibrillation is the initial rhythm present in approximately 40% of non-traumatic sudden-death events. See Homberg, M, et al., “Incidence, duration and survival of ventricular fibrillation in out-of-hospital cardiac arrest patients in Sweden,” Resuscitation, 44(1):7-17, 2000; and Cobb, L, et al., “Changing incidence of out-of-hospital ventricular fibrillation, 1980-2000,” JAMA, 288(23):3008-13, 2000. Electrical defibrillation is the primary modality of treatment of ventricular fibrillation, but evidence is accumulating that its use in the phase or phases of VF generally associated the later time stages of VF, prior to providing ventilation, chest compressions and/or the administration of appropriate medication, is detrimental. Indeed, a number of studies have demonstrated that the probability of successful defibrillation is inversely related to the duration of VF and that immediate countershock was effective as initial therapy for the first several minutes only. See Yakaitis, R W, et al., “Influence of time and therapy on ventricular defibrillation in dogs,” Crit Care Med, 8(3): 157-63, 1980.
The standard of care has been to provide immediate electrical countershock (defibrillation) to terminate the VF rhythm at the earliest possible time. The American Heart Association and the International Liaison Committee on Resuscitation have for many years recommended that three consecutive countershocks be delivered as the initial therapy when VF is present as the initial rhythm in cardiac arrest. See, for example, American Heart Association in Collaboration with the International Liaison Committee on Resuscitation, “Guidelines 2000 for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care: an international consensus on science,” Circulation, 102(8)(Suppl. I):I-136-57 2000 and American Heart Association in Collaboration with the International Liaison Committee on Resuscitation, “International Guidelines 2000 for CPR and ECC: a consensus on science,” Resuscitation, 46(1-3): 1447 2000.
That such therapy might not be optimum therapy in all cases was indicated in a study of CPR for 90 seconds prior to defibrillation. Cobb, L A, et al., “Influence of cardiopulmonary resuscitation prior to defibrillation in patients with out-of-hospital ventricular fibrillation,” JAMA, 281(13):1182-8, 1999. That study demonstrated an increase in survival from 17 to 27% among patients given CPR prior to defibrillation when the response times were over four minutes. A subsequent study also supports those results. Wik, L, et al., “Delaying defibrillation to give basic cardiopulmonary resuscitation to patients with out-of-hospital ventricular fibrillation,” JAMA, 289(11):1389-95, 2003. The results of that study indicated that when ambulance response times were over five minutes, survival to hospital discharge was 22% in patients who had CPR for three minutes prior to defibrillation as compared to 4% survival for those who had defibrillation as the first intervention.
It is a common clinical observation that early VF is “rough” or “coarse” in character or appearance and that it becomes more “smooth” or “fine” over time. Early efforts to quantify this quality and relate it to VF duration examined waveform amplitude as a measure of VF duration. Such attempts have met with only limited success however. See Weaver, W D, et al., “MK. Amplitude of ventricular fibrillation waveform and outcome after cardiac arrest” Ann Intern Med, 102(1):53-5 1985; and Hargarten, K M, et al., “Prehospital experience with coarse ventricular fibrillation: a ten year review,” Ann Emerg Med, 19(2):157-62 1990. The limited success is believed to be a result of the variation in amplitudes measured arising from body habitus, electrode position, electrode conductance, myocardial mass, etc. A number of subsequent attempts have focused on examining the underlying average frequency composition of the waveform as derived from Fourier analysis. See Dzwonczyk, R, et al., “The median frequency of the ECG during ventricular fibrillation: its use in an algorithm for estimating the duration of cardiac arrest,” IEEE Trans Biomed Eng, 37:640-6 1990; Brown, C G and Dzwonszyk, R, “Signal analysis of the human electrocardiogram during ventricular fibrillation: frequency and amplitude parameters as predictors of successful countershock,” Ann Emerg Med, 27(2):184-8, 1996; Berg, R A, et al., “Precountershock cardiopulmonary resuscitation improves ventricular fibrillation median frequency and myocardial readiness for successful defibrillation from prolonged ventricular fibrillation: a randomized, controlled swine study,” Ann Emerg Med, 40(6):563-70, 2002; U.S. Pat. No. 5,077,667, U.S. Pat. No. 5,957,856 and U.S. Pat. No. 6,171,255. While promising, such methods have not been fully developed for clinical use because, for example, the median frequency observed was multiphasic over time, leading to a broad range of time periods possible for a given observed median frequency. Moreover, like waveform amplitude, median frequency can be affected by body habitus, electrode position, electrode conductance, myocardial mass, etc.
Careful study of surface ECG waveforms during VF has led to the consideration that the apparently random activity may in fact be a manifestation of chaos. See, for example, Gray, R A, et al., “Spatial and temporal organization during cardiac fibrillation,” Nature, 392:75-8 1998; Witkowski, F X, et al., “Spatiotemporal evolution of ventricular fibrillation,” Nature, 392:78-82 1998; Witkowski, F X, et al., “Evidence for determinism in ventricular fibrillation,” Phys Rev Lett, 75(6):1230-3, 1995; Garfinkel, A, et al., “Quasiperiodicity and chaos in cardiac fibrillation,” J Clin Invest, 99(2):305-14, 1997; and Hastings, H M, et al., “Nonlinear dynamics in ventricular fibrillation,” Proc Natl Acad Sci USA, 93:10495-9, 1996.
Borrowing from the fields of fractal geometry and nonlinear, chaotic dynamics, several studies addressed the problem of establishing the prior duration of VF in clinical and other settings through use of the scaling exponent (ScE). Callaway, C W, et al., “Scaling structure of electrocardiographic waveform during prolonged ventricular fibrillation in swine,” Pacing Clin Electrophysiol, 2:180-91, 2000; Sherman, L D, et al., “Ventricular fibrillation exhibits dynamical properties and self-similarity,” Resuscitation, 47(2):163-73, 2000; and Lightfoot, et al., “Dynamic nature of electrocardiographic waveform predicts rescue shock outcome in porcine ventricular fibrillation,” Ann Emerg Med, 42:230-241, 2003, the disclosure of which are incorporated herein by reference. The scaling exponent is a measure based on fractal geometry that measures the roughness of the VF waveform. It can be calculated in less than two seconds from a five-second surface recording of the ECG voltages. The scaling exponent has been found to increase over time from a low level of approximately 1.05 to a high level of 1.8 and provides a quantitative measure of the roughness of the VF waveform that is observed to change over time. The scaling exponent has also been shown to be predictive of the probability of successful defibrillation in patients treated with automated defibrillators. Callaway, C W, et al., “Scaling exponent predicts defibrillation success for out-of-hospital ventricular fibrillation cardiac arrest,” Circulation, 103:1656-61, 2001; and U.S. Pat. No. 6,438,419, the disclosures of which are incorporated herein by reference. Recently, the scaling exponent was used to evaluate the effect of performing initial countershock versus starting resuscitation with CPR and/or medication prior to countershock. Menegazzi, J J, et al., “Immediate countershock after prolonged ventricular fibrillation is detrimental,” Circulation, 106(19) (Suppl II):II-192 (abstract) 2002, the disclosure of which is incorporated herein by reference. Those studies have demonstrated that in prolonged VF (that is, VF in which the ScE has progressed to 1.3 or higher), providing CPR and drugs significantly increases survival. The converse of that observation is that defibrillating prior to other interventions in prolonged VF is detrimental and leads to a decrease in survival.
Although progress has been made in developing methods for characterizing ventricular fibrillation, it remains desirable to develop improved devices, systems and methods for characterizing ventricular fibrillation as well as improved treatment devices, systems, methods and protocols for treatment of ventricular fibrillation based thereon.